Fresh water has become a very valuable and scarce resource in recent years not only in arid countries of the Middle East and North Africa regions, but also in many highly populated areas of more developed countries, such as Spain, USA, China and many others. Increases in population and commercial activities have contributed to the depletion of freshwater resources. Desalination, as one of the earliest forms of water production, remains a popular method of water production throughout the world. Desalination typically uses a large amount of energy to remove a portion of pure water from a salt water source. Salt water (feed water) is fed into the process, the result of which is one output stream of pure (fresh) water and another stream of waste water with high salt concentration (brine). Large commercial desalination plants based on fossil fuels are in use by oil-rich countries to supplement their traditional sources of water supply. However, people in many other areas of the world have neither the money nor oil resources to allow them to produce water in a similar manner. Over a billion people today lack access to purified drinking water and the vast majority of these people live in rural areas, where it is very difficult to implement any traditional clean water solution. Difficulties related to the use of fossil fuels could be resolved by switching to renewable resources, such as solar, wind or geothermal energy. Geographical areas where water is needed are in fact rich with renewable energy sources. Thus the obvious way forward is to combine those renewable energy sources with desalination plants. Among various renewable energy resources, the solar energy stands out as the most available, convenient and appropriate energy source for desalination.
The main drawback with the use of solar energy in existing large-scale desalination plants is the resulting low productivity rate and thermal efficiency. However, since solar desalination plants use free energy and therefore have insignificant operational costs, over the long term they are more attractive than conventional approaches. This technology is still suitable even today for small-scale production, especially in remote arid areas and islands, where there is no supply of conventional energy. In addition to cost considerations, there are also environmental concerns about the fossil fuel burning. The coupling of renewable energy sources with desalination processes offers a sustainable, environmentally conscious route for increasing supplies of potable water.
Solar energy can be harnessed for desalination either directly or indirectly. Collection systems, which use solar energy to produce distillate directly in the solar collector, are called direct collection systems, whereas systems that combine solar energy collection systems with conventional desalination systems are called indirect systems. In indirect systems, solar energy is used to either generate the heat required for desalination or generate electricity subsequently used to provide electric power for conventional desalination plants such as multi-effect, multi-stage flash or reverse osmosis systems. Direct solar desalination is primarily suited for very small production systems, such as solar stills. The low production rate is caused by a low operating temperature and near atmospheric pressure of a resulting steam. Numerous attempts have been made in order to produce fresh water by means of solar energy. A simple solar still of a basin type is the oldest method. A solar still is a simple device that can be used to convert saline or brackish water into drinking water. Solar stills use exactly the same processes, which in nature generate rainfall, namely evaporation and condensation: a transparent cover encloses a pan of saline water that is first evaporated by the trapped solar energy within the enclosure and then condensed on the inner face of the sloping transparent cover. This distilled water is generally potable; the quality of the distillate is very high because all the salts, inorganic and organic components, and microbes are left behind in the bath. One of the problems that negatively influence the still performance is the direct contact between the collector and the saline water, which may lead to corrosion and scaling in the still. The biggest issue for the solar stills however are their rather low efficiency and water production rate: a typical production rate of a solar still is about 4 L/m2/day or less.